Agree with Lavar. Unsafety on block means diddly squat if you can confuse your opponent and make them panic hold. And it’s a setup you won’t be using all the time anyway, since you don’t want to be obvious.
Assuming your opponent doesn't do anything which isn't that bad since most won't know exactly how to react. If done during a stun set up or in general your opponent will likely try to mid hold to avoid the 4P214P+K cancel or they'll likely hold in general so you should be at advantage anyway.
I found a simple way for Mai to get behind her opponent: Use 4P and then immediately press 9P for her to jump behind the opponent. You'll have to move a bit into the background to get the BT effect damage and attack wise tho. XD
The same can also be done from 3P+K on normal hit, except in this case you'll have to move a bit into the foreground so you can get attacks into BT status. Be aware that the opponent can't high hold for this to work which means you'll likely get more use from this when performing 4P. Furthermore, if done when your opponent mid holds or low holds, you'll go over their head and directly behind them too. Neat! :-D This will only work from the 4P method however, 3P+K will only let you jump over from a low hold. Oh and a quick guaranteed go to in this situation would be 6PP214P or 6PP236K.